1. Field of the Invention
The present invention relates to a controller for a delay variation buffer, and in particular to a controller and control method suitable for a delay-variation buffer absorbing delay variation of cells received from ATM (asynchronous transfer mode) network and transferred to STM (synchronous transfer mode) network.
2. Description of the Related Art
In a network composed of an ATM network connecting two STM networks that both uses existing network equipment to transfer data in a period of constant frame, a circuit emulation service is needed at an interface between ATM and STM networks to perform bi-directional conversion between ATM cell and STM frame. A circuit emulator providing the circuit emulation service (hereafter, abbreviated as CE) is implemented by emulating STM on ATM line.
Since ATM cells may be transferred through different paths across an ATM network, variations in delay or jitter occur to cells that were nominally spaced prior to transfer across the ATM network. In order to generate STM frames from asynchronously arriving cells and stably and reliably transmit them to the STM network, the CE is provided with a buffer for buffering ATM cells arriving with variations in delay and a delay-variation absorbing controller.
There have been proposed several delay-variation buffer controllers aiming at absorbing the variations in delay to avoid a substantial reduction in the quality of service.
For example, Japanese Patent Application Unexamined Publication No. 4-331529 discloses a delay-variation buffer controller that controls a delay-variation absorbable width based on a calculated cell loss ratio.
However, such a delay-variation buffer controller can be applied to only the case where ATM cells permitted to be discarded are converted into STM frames. In other words, a delay-variation control cannot be successfully performed unless ATM cells are permitted to be discarded. Since ATM communication is characterized in that a network can be constructed independently of type of service, it is preferable that the delay-variation buffer control is applied to not only ATM cells that are permitted to be discarded but also ATM cells that are not permitted to be discarded.
As another prior art, Japanese Patent Application Unexamined Publication No. 9-102772 discloses an ATM/STM converter in which readout of cells from a delay-variation absorbing buffer is controlled based on detected cell delay variation on ATM line. Such a buffer control can avoid increasing data delay caused by the capacity of the buffer increasing more than necessary in communication environment with wide variation in delay.
However, this prior art needs to measure the amount of cell variation by sending test cells before actual data transmission. Therefore, the detected cell delay variation does not reflect an actual network operation status, which cannot achieve proper cell delay variation control.